1. Field of the Invention
This invention relates to a process for the automatic control of the step-by-step advance of a foil sheet or the like, as well as an arrangement for carrying out said process.
2. Description of the Prior Art
As a continuously operating machine in the case of this process, for example, a printing machine, a punching machine or a similar machine may be used. This process can especially advantageously be used in the case of a printing machine having an approximately crescent-shaped printing roller and inking roller, in which case the degree of contact of the two rotating cylinders for the printing of different paper formats can be continuously adjusted in the direction of the roller circumference. The degree of contact of the two rollers is adjusted in such a way that these rollers, at the circumference, while the machine is operating during the so-called contact phase, have contact for a period of time that is as long as the length of the paper format to be printed. The part not used for machining or printing, such as the part of the roller circumference of a printing machine that is not used for printing, of the moving, preferably rotating machine parts will be called a vacant channel. In this vacant channel phase, the paper sheet is at rest, and the format to be printed, at the start of the contact phase, must move correctly on the printing roller with respect to position and synchronously with respect to the printing picture.
In the following, this invention relates to printing machines but is basically also suitable for the control of other foil sheets to be machined that may comprise any material, such as sheet metal or any similar material.
In the case of known processes of the initially mentioned type, usually no satisfactory result is obtained because the respective conveyed format of the foil sheet can be machined, for example printed, only partially and not precisely with respect to position. The reason is that the transport of the paper sheet in this case takes place by frictional connection with the pair of rollers which achieves a precisely synchronous operation with the roller surfaces, but no satisfactory precision as far as the position of the format with respect to the printing picture is concerned.
The paper must be accelerated starting from the rest position. This takes place at the start of the contact phase of the two rollers. Since physically and in practice, an infinite acceleration is not possible, the above-mentioned varying offset of the format with respect to the printing picture will occur in each case according to the random conditions that exist caused by a slip which first is not constant and secondly depends on the speed.
Naturally, no printing can take place during the acceleration phase. After the end of the contact phase, the paper, according to the speed, will follow for so long until the kinetic energy is used up by means of suitable measures (stoppers, etc.). Thus neither at the start of the transport, nor at the end of the transport, reproducible conditions exist that are required for achieving the desired results. In addition, because of the fact that the whole transported paper format is printed only partially, printing can, for example, not take place from fold to fold in the case of a Leporello-type folded reel paper.
The degree of contact of the two cylinders must necessarily be smaller than the transported format that is to be printed.
It is therefore required to accelerate the paper sheet starting from the rest position by means of a driving device until it moves synchronously with the printing roller. It is true that in this case the paper sheet is moved in forward direction but it cannot yet be printed because it does not yet move synchronously with the printing machine. This phase will be called lead run.
During the contact phase of the rollers, the paper sheet must be moved synchronously with respect to the roller because otherwise the paper will tear. After the end of the contact phase, at the start of the vacant channel, the paper sheet must be stopped. Because of the inertia of the moving parts, these parts, despite extensive braking, continue to move a little further in forward direction. This phase will be called overrun.